1. Field of the Invention
This invention relates generally to plasma density measurement and particularly to a device for measuring the plasma density profile in aerodynamic flow fields about supersonic space vehicles or test models.
2. Description of the Prior Art
It is important in the field of plasma measurement to be able to measure the plasma density profile about a space vehicle traveling at supersonic speeds during reentry or of a test model under simulated reentry conditions in order to predict or determine the effect of the plasma sheath on radio-wave transmission, reception, and diffraction. A plasma sheath is an envelope of ionized gas that surrounds the space vehicle or test model moving through an atmosphere at a supersonic velocity.
Heretofore, a number of devices have been used to measure the plasma density in aerodynamic flow fields about supersonic bodies. These include microwave and radio frequency probe devices. For small test models, like those contemplated for use with the present invention, such devices are too large to be carried abroad the model. In addition, these devices do not provide useful information about the plasma density profile.
As background information, one such device is disclosed in U.S. Pat. No. 3,750,013, filed Oct. 21, 1970, by Rotman et al., entitled, "Microstrip Plasma Probe", granted July 31, 1973. Rotman et al. disclose a microstrip cavity type transmission line having a lossy center conductor which provides attenuation and reflection coefficient information in response to the disturbing effects of conducting plasma proximate to the transmission line. Equivalent conductance and susceptance of the plasma is determined by using the foregoing information to solve standard transmission line formulas.
Thus, the information gathered by the device of Rotman et al. will allow calculation of certain electrical properties of the plasma very close to the surface of the space vehicle or test model. The information gathered is not useful to calculate or predict the plasma density as a function of distance away from the surface of the space vehicle or test model i.e., a measure of the plasma density profile.
Consequently, there is a need in the prior art for a device to provide information useful for determining the plasma density profile, but yet be small enough to be carried abroad or be mounted flush to the surface of a test model.
A simple device for measuring plasma density which can be carried aboard or be mounted flush to the surface of a test model is a dc flush mountable double probe. These devices have been used to measure the plasma density in aerodynamic flow fields about test models, but they suffer from the defect that they can only provide information about the plasma density which is an average of the values close to the surface of the test model. The information gained in this situation is not a measure of the plasma density profile.
Hence, there is a need in the prior art to modify the standard dc flush mountable double probe to permit a measurement of the plasma density profile about a test model in an aerodynamic flow field.
The representative prior art, as outlined hereinabove, include many advances in plasma measurement techniques; however, insofar as can be determined, no prior art device incorporates all of the features and advantages of the present invention.